1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-055716, filed on Mar. 6, 2008, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to an optical transmission system and, more particularly, to a coherent receiver and a method for adjusting the coherent receiver.
2. Description of the Related Art
Optical transmission systems, which use optical fiber as a transmission medium, play an essential role in trunk communication networks, for their excellent long-distance and large-capacity transmission performances. In many of optical transmission systems provided for practical use, a signal to be transmitted is superposed onto signal light by intensity modulation. Specifically, a digital binary (I/O) signal is assigned light ON/OFF. In part of the latest optical transmission systems in which the transmission rate is more than 10 Gb/s, a scheme is used by which the superposition of a signal is performed by using phases, not the intensities. In this case as well, conversion to a signal with two intensity levels is performed by optical processing at a receiving terminal before the signal is received.
Optical communication using a two-intensity-level signal is easy to implement but is not always the best in performance. Nonetheless, such systems of two-intensity-level signal type have continued to be the main stream, and a factor for making this possible is that optical amplifiers have become commercially practicable. Optical amplifiers, which can linearly amplify signal light directly, became commercially practical in the first half of the 1990s. However, actually, various other different schemes had been studied earlier. Coherent optical transmission schemes are among these schemes.
The coherent optical transmission schemes, by which signal light is combined with local-oscillator (LO) light at a receiving terminal and their beat component is detected by an optical-to-electrical (OE) converter, had been vigorously studied in spite of the structural complexity before optical amplifiers became commercially practical. The reason is that the coherent optical transmission schemes accomplish excellent reception sensitivity. Although the excellence in reception sensitivity has no longer been regarded as an advantage large enough to compensate for the structural complexity since optical amplifiers became commercially practical, the coherent optical transmission schemes still continue to draw attention from different points of view.
As the signal rate of an optical communication system increases to 10 Gb/s, 40 Gb/s, and so on, waveform distortion more easily occurs in signal light due to the interaction between bits in transmission fiber or other places. As a countermeasure, waveform correction processing and the like are needed at an electrical stage after reception. In this case, only information on the intensity of the signal light, provided from an OE converter, is not sufficient in terms of information quantity. In this respect, coherent reception schemes draw attention because information on phases is also obtainable at the same time.
Moreover, as the signal rate increases, more technical problems arise with the fact that a serial signal, in which signals are simply time-division-multiplexed, is processed directly. As a measure for easing the processing rate, development from a two-level signal to a multilevel signal has also been sought. When a multilevel signal is used, it is necessary to electrically perform complex coding/decoding processes involved. The superiority of the coherent reception schemes is also recognized from such a point of view.
As yet, the reported results of experimental studies have not exceeded the feasibility verification level because, in part, of a performance problem with electronic parts for waveform correction processing and decoding processing. However, a variety of studies are being pursued, such as dense wavelength division multiplexing in a WDM transmission system (see K. Kikuchi, “Phase-diversity homodyne detection of multilevel optical modulation with digital carrier phase estimation,” IEEE Journal of selected topics in quantum electronics (2006), Vol. 12, No. 4, pp. 563-570), large-scale wavelength dispersion compensation in a 40-Gb/s system (see C. Laperle et al., “Wavelength division multiplexing (WDM) and polarization mode dispersion (PMD) performance of a coherent 40 Gbit/s dual-polarization quadrature phase shift keying (DP-QPSK) transceiver,” OFC/NFOEC2007, paper PDP16, 2007), and application to a 100-Gb/s signal (see C. R. S. Fludger et al., “10×111 Gbit/s, 50 GHz spaced, POLMUX-RZ-DQPSK transmission over 2375 km employing coherent equalization,” OFC/NFOEC2007, paper PDP22, 2007).
Moreover, JP2004-147323 also discloses a method for adjusting an optical receiver by which the signal-to-noise ratio (SNR) of a heterodyne beat signal is improved. According to the optical heterodyne detection system disclosed in JP2004-147323, an attenuator, which attenuates an input signal, is provided before an optical coupler, which combines the input signal with a LO signal, and an optimum attenuation level is calculated based on a base measured value and sample measured values, wherein the base measured value is the value of an optical detected signal derived only from the LO signal without the input signal being provided, and the sample measured values are the values of optical detected signals derived from the input signals attenuated at different attenuation levels. Moreover, a minimum attenuation level is set such that the input signal will have as large a value as the DC detection voltage limit of an optical detector, thereby preventing the optical detector from saturating.
However, in a coherent optical receiver, input is signal light suffering various waveform distortions or signal light in which individual bits have different amounts of energy. Therefore, the amplitude of the input signal varies with waveform-distorting factors even if the average intensity of the input signal light is fixed. As an example, a description will be given of changes in the waveforms of signal light due to wavelength dispersion.
FIG. 1A is a graph showing signal waveforms output from a transmitter, FIG. 1B is a graph showing signal waveforms after transmission over a 20-km fiber line, FIG. 1C is a graph showing signal waveforms after transmission over a 40-km fiber line, and FIG. 1D is a graph showing signal waveforms after transmission over a 200-km fiber line. Here, it will be described how waveforms (equivalent to 128 bits in total) change when a 40-Gb/s RZ-DQPSK signal is transmitted over an ordinary SMF transmission line (wavelength dispersion value: 17.0 ps/nm/km).
Referring to FIG. 1A, when the signal is in a state of being output from the transmitter, there is no difference in level between the bits. The signals shown in FIGS. 1A to 1D all have the same average intensity. However, it can be understood that, as wavelength dispersion accumulates and causes interactions between preceding and following bits, the waveforms gradually gain local peaks, resulting in the signal whose amplitude is enlarged on the whole.
When an optical signal having such waveform distortions is received, an optical receiver needs to perform compensation for signal waveform distortion through digital signal processing at an electrical stage. In this event, an electrical signal obtained by optical-to-electrical conversion is converted from an analog signal to a digital signal by an analog-to-digital converter (AD converter). To achieve high-sensitivity reception characteristics by performing waveform distortion compensation with high precision, the amplitude of the signal to be input to the AD converter needs to be adjusted so as to uniformly fall into the input dynamic range of the AD converter.
In this event, if the input signal light is only degraded in terms of optical SNR and no great waveform distortion occurs as in cases of existing optical communication systems, an optimum operating environment can be provided in any system by fixing the average intensity of the input signal light. That is, it is possible to allow the input dynamic range of the AD converter to be uniformly used. However, if waveform distortion occurs and, as a result, the signal amplitude varies with systems, then it is necessary to perform optimization for each case.
In cases of coherent optical receivers used for the purpose of compensating for signal waveform distortion through digital signal processing at an electrical stage, there is a possibility that input signal light has various distortions. Accordingly, an optimum operating environment cannot be realized only by simply controlling the intensity of the input signal light to a certain fixed value as described above. That is, if the receiver is optimized suitably to a state of small waveform distortion, a signal input to the AD converter, in an area where the waveform distortion is large, exceeds the input dynamic range of the AD converter, resulting in accurate demodulation being impossible. On the other hand, if the receiver is optimized suitably to waveforms having large distortions, only part of the ability of the AD converter is used when signal light suffering small waveform distortion is input, resulting in the reception performance being degraded.